Cancellation

• Thread cancellation is the task of terminating a thread before it has completed.
  • For example, if multiple threads are concurrently searching through a database and one thread returns the result, the remaining threads might be canceled.
  • Another situation might occur when a user presses a button on a web browser that stops a web page from loading any further.
• A thread that is to be canceled is often referred to as the target thread. Cancellation of a target thread may occur in two different scenarios:
  1. Asynchronous cancellation. One thread immediately terminates the target thread.
     • The difficulty with cancellation occurs in situations where resources have been allocated to a canceled thread or where a thread is canceled while in the midst of updating data it is sharing with other threads.
     • Often, the OS will reclaim system resources from a canceled thread but will not reclaim all resources. Therefore, canceling a thread asynchronously may not free a necessary system-wide resource.
  2. Deferred cancellation. The target thread periodically checks whether it should terminate, allowing it an opportunity to terminate itself in an orderly fashion.
     • With deferred cancellation, in contrast, one thread indicates that a target thread is to be canceled, but cancellation occurs only after the target thread has checked a flag to determine if it should be canceled or not.
     • This allows a thread to check whether it should be canceled at a point when it can be canceled safely. Pthreads refers to such points as cancellation points.